1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device.
2. Description of the Related Art
A flash memory and a ferroelectric memory are well-known as nonvolatile memories capable of storing information even after a power supply is turned off.
The flash memory includes a floating gate that is embedded in a gate insulating film of an insulated gate filled effect transistor (IGFET), and stores information by accumulating electric charges indicating recording information. However, there is a drawback in that the flash memory requires relatively high voltage because it is necessary to flow the tunnel current to the gate insulating film at the time of writing and erasing the information.
On the other hand, the ferroelectric memory, which is also referred to as FeRAM (Ferroelectric Random Access Memory), stores information by utilizing the hysteresis property of a ferroelectric film provided in a ferroelectric capacitor. The ferroelectric film causes polarization in response to the voltage applied between upper and lower electrodes of the capacitor, and spontaneous polarization remains even after the voltage is removed. When the polarity of the applied voltage is reversed, the polarity of the spontaneous polarization is also reversed. By bringing the direction of the polarity to correspond to “1” and “0”, the information is written in the ferroelectric film. The voltage required for the FeRAM to carry out writing is lower than that for the flash memory to carry out writing. In addition, there is also an advantage in that the FeRAM is capable of writing at a higher rate than the flash memory.
In a process of manufacturing the FeRAM, a silane (SiH4) gas or a TEOS gas is used as a deposition gas at the time of forming an interlayer insulating film made of oxide silicon which is formed in the upper portion of the capacitor by a chemical vapor deposition (CVD) method. These gases release hydrogen to a deposition atmosphere by decomposition at the time of deposition. However, it is known that when the capacitor dielectric film comes in contact with reductant such as hydrogen, it causes shortage of oxygen because oxygen in the film is deoxidized, thereby causing the deterioration of ferroelectric property, such as residual amount of polarized electric charges, of capacitor dielectric film. For example, when the capacitor dielectric film is heated in an atmosphere with hydrogen having partial pressure of 40 Pa, the ferroelectricity of the capacitor dielectric film is substantially lost, thereby causing the remarkable deterioration of hysteresis curve.
In addition, even when the capacitor dielectric film is heated in a situation with water in the vicinity, the ferroelectricity is deteriorated because, similar to the above case, water serves as the reductant to deoxidize the capacitor dielectric film.
In this manner, the capacitor dielectric film is deteriorated under the circumstance that heat and reductant are co-existed.
Therefore, this type of FeRAM requires a protective film to protect the capacitor from a reducing atmosphere.
For example, as set forth in Japanese published unexamined application No . 1997-307074, a capacitor is covered by an oxide silicon film formed by a sputtering method in which water is hard to be involved, and the oxide silicon film is used as a protective film.
In addition, as set forth in Japanese published unexamined application No. 2003-273325, as shown in its FIG. 2, metal interconnects in the upper portion of the capacitor are covered by a protective film such as a titanium oxide film or an alumina film so that water can be prevented from coming in contact with the metal interconnects so as not to generate hydrogen.
On the other hand, as set forth in Japanese published unexamined application No. 2000-164817, as shown in its FIG. 3, in an iridium film or the like is formed so as to adjust stress of which a capacitor is received, and a protective film made of a silicon nitride film and a silicon oxynitride film is formed thereon.
As set forth in Japanese published unexamined application No. 1999-126883, as shown in its FIG. 1, a TaSiN film having a hydrogen gas blocking property is formed on the upper electrode of a capacitor so that the permeation of hydrogen into a capacitor dielectric film can be prevented.
Other than the above-described methods, by utilizing methods such as a DC sputtering method, an RF sputtering method, an ion beam deposition method, a plasma CVD method, or a sol-gel method, a titanium or aluminum nitride film or ferroelectric film can be formed, and these films can be a protective film to hydrogen.
It should be noted that the related art to the present invention are also set forth in Japanese published unexamined application No. 2004-95861 and “Preparation of SiNx Passivation Films for PZT Ferroelectric Capacitors by Catalytic Chemical Vapor Deposition” written by Toshiharu Minamikawa et al, which is available online on http://www.irii.go.jp/theme/h12/pdf/study02.pdf.